Jim,
Yes, you're right, I hear almost the identical question regularly. And my answer is always the same: A 65-pound compound is more than adequate. The bow will do the job. The person behind it is the only concern. Can you put an arrow in the boiler room? If so, you've got a dead elk on your hands.
Just one thought on range: You say you practice out to 55 yards. That's great, because it helps fine-tune your shooting skills. But I personally would consider that a very long shot on elk. Perhaps in the open sage such a shot would be feasible, but most elk hunting takes place in timber, where you cannot judge obstructions such as limbs, twigs, and leaves. And generally you have little time for range estimation. So, I would say in a real-life elk hunting situation, you should try for shots of 20 to 30 yards. That's where you'll make your sure, clean kills.
The only thing you can do to acclimate to high elevations is to spend time at high elevations. Thus, I would recommend you arrive in your hunting area three or four days before you really start hitting it hard, especially if you're going to be hunting at 12,000 feet in Colorado. You could be sucking wind there for awhile. I do think it helps to arrive in good shape. It will take you less time to adapt. It sounds like you have a good training program, but I would suggest you kick the running up to 30 miles or so per week. That will give you just that much more endurance. And one thing I will guarantee you -- you can't be in TOO good of shape.
Have fun,
Dwight Schuh